The production of ethers by the reaction of an isoolefin with an alcohol is well known and is practiced commercially. This highly selective reaction is also used to remove isoolefins, especially isobutylene, from mixed hydrocarbon streams such as the C.sub.4 streams produced in an ethylene-producing steam cracking plant or in an FCC unit. Much attention has been focused on ether production due to the rapidly increasing demand for lead-free octane boosters for gasoline such as methyl-tertiary-butyl ether (MTBE).
A detailed description of processes, including catalysts, processing conditions and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in the following article; Stinson, New Plants, Processes Set for Octane Booster, CHEMICAL AND ENGINEERING NEWS, June 25, 1979 at pp. 35-36. The preferred etherification zone is described in a paper presented at the American Institute of Chemical Engineers 85th National meeting on June 4-8, 1978 by F. Obenaus et al., entitled Huls-Process: Methyl Tertiary Butylether.
More recently, there has been an increased commercial interest in the use of ethyl-tertiary-butyl ether (ETBE) as a lead-free octane booster for gasoline. Note, for example, the following publications: Iborra et al., Getting The Lead Out With Ethyl t-Butyl Ether, CHEMTECH, Feb. 1988, pp. 120-122; and Berbonic, ETBE: Ethanol's Motor Fuel Hope?, CHEMICAL BUSINESS, Oct. 1988, at pp. 38-39, and the paper presented at the DeWitt Petrochemical Review, Houston, Tex. Mar. 28-30, 1989 by Nierlich et al., entitled Huels/UOP Technology for ETBE/MTBE Production.
Ethyl-tertiary-butyl ether has long been recognized as a suitable blending cosolvent for hydrous ethanol in gasoline stocks. See U.S. Pat. No. 4,207,076. ETBE can be blended into a fuel gasoline at about a 10 to 20 volume percent level, more usually about 9 to 12%, in which the fuel comprises about 70 to 84% gasoline and 5 to 20% of 95% ethanol, i.e., grain alcohol. ETBE solubilizes grain alcohol in gasoline in all proportions thereby allowing a wide latitude in the precise amount of ethanol which can be blended with the gasoline. In addition, the presence of ETBE in the blend considerably increases its octane rating, both motor and research.
ETBE and MTBE are produced by reacting isobutene with either ethanol or methanol, resulting respectively in the formation of ETBE or MTBE. The reaction normally is conducted in liquid phase with relatively mild conditions. While mixed butene streams can be employed, only the tertiary olefin, isobutene, reacts as the conditions employed. The isobutene can be obtained from various sources, such as naphta cracking, catalytic cracking, etc. For example, refer to, Muddarris et al., Now, MTBE from Butane, HYDROCARBON PROCESSING, Oct. 1980, at pp. 91-95. This highly selective reaction can also be used to remove isobutene from mixed hydrocarbon streams such as the C.sub.4 streams produced in steam cracking plants which produce ethylene.
The resulting reaction product stream contains the desired MTBE or ETBE, as well as unreacted isobutene and other C.sub.4 hydrocarbons and methanol or ethanol. A problem that persists has been the separation of the unreacted alcohol from the ETBE or MTBE product. At equilibrium conversion, considerable alcohol remains in the reactor effluent, is difficult to remove by simple distillation, and may form troublesome azeotropes with ETBE and MTBE.
Several solutions to this problem have been disclosed. For example, U.S. Pat. No. 3,726,942 discloses the removal of methanol from the unreacted hydrocarbons withdrawn from an MTBE reaction zone through the use of molecular sieves. U.S. Pat. No. 4,322,565 discloses the removal of alcohols from the effluent of an etherification reaction zone through absorption of the alcohol on solid calcium chloride. The reference also discloses that the absorbent can be regenerated with a hot hydrocarbon stream which can be charged to the etherification reactor to recycle the alcohol. This absorption step is a bulk removal which replaces fractional distillation of the etherification reaction zone effluent.
U.S. Pat. No. 4,371,718 discloses that the effluent of the MTBE reactor may be fractionated to yield a C.sub.4 stream which is then passed through methanol adsorption zones. The adsorption zones are regenerated with hot hydrocarbons which are then passed into the MTBE reactor. The methanol-free C.sub.4 stream is then passed into an alkylation zone.
U.S. Pat. No. 4,440,963 discloses a process wherein the fractionation of the methanol- or ethanol-containing methyl-(or ethyl)-tertiary-butyl ether reactor effluent is improved by employing 2-methylpentane to azeotrope methanol or ethanol overhead, or by using 1,1,3-trichloro-1,2,2-trifluoroethane (F113) to azeotrope methanol overhead, leaving a substantially pure MTBE or ETBE bottoms.
U.S. Pat. No. 4,490,563 discloses a process wherein MTBE is recovered from an ether-containing effluent by fractionation. In one aspect of the process of the above-described patent, when driers are not used to pretreat the hydrocarbon feed to MTBE manufacture, i.e., water-containing feeds, a separate water-methanol phase occurs in the fractionation overhead which is separately processed in a methanol fractionator and the water recovered is used to water wash the separated hydrocarbon phase from the overhead while methanol is recycled to MTBE manufacture.
U.S. Pat. No. 4,302,298 discloses a process for isolating methyl tert-butyl ether contained in the reaction product of methanol with a C.sub.4 hydrocarbon cut containing isobutene, comprising fractionating said reaction product by introducing it at an intermediate point of a distillation zone, recovering methyl tert-butyl ether at the bottom thereof and, at the top thereof, a mixture of C.sub.4 hydrocarbons with methanol. The C.sub.4 hydrocarbons mixture is washed with water to form a water-methanol fraction and a C.sub.4 hydrocarbon fraction, a portion of which is fed back as reflux to the top of the distillation zone and another portion discharged. The remaining water-methanol mixture is fractionated by distillation.
U.S. Pat. No. 4,334,964 discloses a method for separating mixture of reaction products produced in the catalytic etherification of a lower isoolefin with methanol which comprises adding water to said reaction product to form a hydrocarbon phase containing the product ether and an aqueous phase containing methanol and a tertiary alcohol. The products are thereafter separated by distillation and the methanol is recycled to the reaction feed stream.
The processes described above provide, by methods including distillation, absorption and adsorption, for the recovery and recycle of the monohydroxy alcohols, e.g., methanol and ethanol, to the etherification reactor feed. These separation steps are effective to avoid alcohol losses with the product streams. However, some separation steps, particularly those which provide for the recovery of the monohydroxy alcohols from the azeotropic mixtures of unreacted alcohols and hydrocarbons, can unduly increase the overall process complexity. Accordingly, processes for the etherification of monohydroxy alcohols and isoolefins are sought which can provide for the recovery and recycle of unreacted alcohols without complicated separation steps. Such processes are especially desired in the production of ETBE wherein the azeotropic ethanol content with hydrocarbons is typically lower than the corresponding azeotropic methanol content in MTBE production. Moreover, since ethanol is often obtainable in azeotropic form, i.e., about 5 vol. % water, processes are sought for the production of ETBE which can utilize azeotropic ethanol as a feed source.